Transparent molded articles for optics have hitherto been obtained from acrylic resins taking advantage of their transparency, satisfactory fluidity, small birefringence, and the like as described, e.g., in Japanese Patent Application (OPI) No. 131654/81 (the term "OPI" as used herein means "unexamined published Japanese patent application"). Acrylic resins, however, have disadvantages, such as poor heat resistance (about 70.degree. C.), low impact resistance, and liability to warping on contact with moisture. In order to eliminate these disadvantages, it has been proposed to use a polycarbonate resin having a viscosity-average molecular weight of from 15,000 to 18,000 as a molding material for discs, lenses, etc. as disclosed in Japanese Patent Application (OPI) No. 180553/83. The above-described polycarbonate resin still has problems, such as significant birefringence, that is a bar to use as optics, so that the application to be made of it is limited.
One of important subjects to consider in practical application of optical materials, and chiefly materials for optical discs is reduction of a noise level of a base itself. It has been elucidated that the noise level in question depends on birefringence of incident beam , inclusive of oblique incident beam as reported, e.g., in Optics, Vol. 15, No. 5 (Oct., 1986), 441-421, and Articles of '86 Photo Memory Symposium, 33-38. That is, reduction of birefringence of vertical incident light does not always correlate to changes of birefringence of oblique incident beam. In the case of polycarbonate resins, this difference is particularly conspicuous. Therefore, it is important to accomplish reduction of birefringence in a wide range of incident angle.
Many proposals have been made to reduce birefringence by using a modified aromatic polycarbonate or a compostion of an aromatic polycarbonate and other resins as taught, e.g., in Japanese Patent Application (OPI) Nos. 19630/86, 19656/86, 18466/87, 20524/87, and 108617/86, and Functional Materials, 21-29 (Mar., 1987). All of these proposals have for their object reduction of birefringence of vertical incident beam , and there is no reference to birefringence of oblique incident beam . Therefore, they do not reach full satisfaction from the standpoint of noise level reduction in a wide angle range of beam. A further important consideration is that a composition prepared by mere mixing of polymers or a composition comprising a copolymer but necessarily containing a large quantity of homopolymers tends to have a coarse disperse phase. As a result, if a micro disperse phase grows to a particle size exceeding 0.5 .mu.m, for example, 3 .mu.m, a difference of refractive index is produced microscopically, i.e., among individual beam paths and the interface of dispersed particles becomes a source of scattering due to the difference in refractive index even in a system having its measured birefringence reduced to zero, resulting in optical non-uniformity of the system and thereby noise making.
Hence, the conventional modified polycarbonates involve basic drawbacks for use as a molding material for optics, particularly a base of photo discs.